YALE ECONOMIST ROBERT SHILLER delivers his forecast for U.S. housing with a scholarly diffidence that only slightly mutes his stark message: The market is in the throes of a bubble of unprecedented proportions that probably will end ugly.

Such unsettling talk is cheap, of course, especially from a tenured academic, and many sources, including Barron's, have wrongly predicted housing's downfall several times in the past few years. But the Ivy League professor's forecasts of coming trouble have been right before. His best...